Sensor element, in particular a planar gas sensor element

ABSTRACT

A sensor element, in particular a planar gas sensor element, having a sensor structure is described, which is heatable by a heater structure. A first spacer layer is provided between the heater structure and the sensor structure, the spacer layer having a first recess in the area of the heater structure into which a first inlay, which electrically insulates the heater structure from the sensor structure, is inserted.

FIELD OF THE INVENTION

The present invention relates to a sensor element, in particular aplanar gas sensor element, such as a lambda probe or a nitrogen oxidesensor, that includes a solid electrolyte and a heater structure.

BACKGROUND INFORMATION

Planar gas sensor elements (“lambda probes”), may be heated using aheating device having a heater structure that is incorporated into amultilayer ceramic layer structure. A main function of the heating is tostabilize the sensor element signal. German Published Patent ApplicationNo. 199 06 908 (the '908 application) describes a heater structuredesigned as a platinum resistance conductor in a meandering patternbetween two ceramic layers in the hot area of the gas sensor element,i.e., in the area in which the measuring and reference electrodes aresituated, and which is exposed to the gas to be analyzed.

In the case of ceramic gas sensor elements based on a solid electrolytemade substantially of zirconium dioxide, it is also necessary toelectrically insulate the heater structure from the ionic conductors,i.e., solid electrolytes, provided in the area of the actual sensorstructure. To do so, either a printed heater structure as described inthe '908 application is embedded between two layers of aluminum oxide,likewise printed, and having a thickness of approximately 20 μm to 50μm, or the heating device is sintered or glued over the entire area ofone side of a sensor element having a heater structure already embeddedbetween two ceramic films.

One disadvantage of these two methods, however, is the mechanicalstresses which occur in the sensor element during operation and/ormanufacture and are caused mainly by differences in the thermalexpansion coefficients of the materials used as well as thecomparatively great heat flow to the side of the sensor element facingaway from the sensor structure.

An object of the present invention is to provide a sensor element havinga heater structure having the lowest possible capacitive electriccoupling to the respective sensor structure and/or the ionic conductor,i.e., solid electrolyte, used there. In addition, an object of thepresent invention is also to supply the heat generated by the heaterstructure to the sensor structure as much as possible while at the sametime preventing mechanical stresses within the sensor element.

SUMMARY OF THE INVENTION

The sensor element according to the present invention has the advantageover the related art that the heater structure has only a low capacitivecoupling electrically with respect to the sensor structure so that theactual sensor function is virtually unimpaired electrically by theheater structure apart from the desired heating effect.

In addition, the design of the sensor element according to the presentinvention achieves the result that mechanical stresses within the sensorelement are suppressed as much as possible and the sensor structuresituated in the vicinity of the heater structure is heated effectivelyand rapidly by the heater structure.

The hot area of the heater structure may be inserted between twoelectrically insulating inlays, which are preferably made of aluminumoxide. In this way the hot area of the heater structure is integratedinto the sensor element, and the electrically insulating inlays togetherwith the heater structure form an insulation body surrounded completelyor partially by other layers of the sensor element, usually composedessentially of zirconium dioxide.

Electrically insulating intermediate layers may be provided between theinsulation body formed by the electrically insulating inlays and theheater structure embedded therein and the adjacent zirconium dioxidelayers to counteract shrinkage of the two layers during sintering, byhaving the electrically insulating inlay be composed of aluminum oxideand the adjacent layer be composed of zirconium dioxide.

With regard to the desired reduction in capacitive coupling, it is alsoadvantageous when the electrically insulating inlay and the providedsecond inlay each have a thickness of 100 μm to 1000 μm, e.g., 200 μm to500 μm.

The first spacer layer may lateral surround the first recess, whichaccommodates the electrically insulating first inlay, in the form of aclosed frame. The second spacer layer may also laterally surround thesecond recess accommodating the second inlay, again in the form of aclosed frame. This forms an insulation body composed of the inlays andthe heater structure embedded therein, the entirety being completelyenclosed by the substrate, the frame-like spacer layers in some areas,and an additional zirconium dioxide layer that may be provided betweenthe insulation body and the actual sensor structure.

Intermediate layers may be provided between the inlay and the adjacentlayer. These may be composed of zirconium dioxide, and may have a lowsintering activity and may have a low sinter density so that they remainporous after sintering and may act as stress equalizing layers.Alternatively, one or both intermediate layers may also be designed asmechanical stress-absorbing layers, which entails a sufficient adhesionand cohesion between the inlay and the intermediate layer, on the onehand, and between the intermediate layer and the side of theintermediate layer facing away from the inlay on the other hand. In thecase of the stress-equalizing layer, a magnesium aluminum spinel, suchas MgAl₂O₄, or barium hexaaluminate has proven especially suitable asthe material for the intermediate layer, or in the case of thestress-absorbing layer, a mixture of zirconium dioxide and aluminumoxide has proven particularly suitable.

The desired low capacitive coupling is further enhanced by having thethickness of the electrically insulating first inlay and also thethickness of the optional second layer comparatively large due to theintermediate layers without resulting in deformations or cracks in thesensor element during manufacture, sintering, or operation, or atalternating temperatures, due to the lower thermal expansion of aluminumoxide in comparison with zirconium dioxide.

Comparative measurements have shown that the measures described abovemake it possible to reduce capacitive coupling by a factor of at least 5to 10.

It is also advantageous that improved heat transfer from the heaterstructure into the sensor structure is achievable in that the rear areaof the sensor element in the area of the inlays. The side of the sensorelement laterally opposite the heater lead wire is surrounded by thespacer layers designed as a frame. The zirconium dioxide, which has poorthermal conductivity, provided in this rear area thus prevents unwantedheat dissipation. Moreover, this rear area may now have a lateralextension, as defined by the width of the frame, of significantlygreater than 300 μm, e.g., 500 μm to 2000 μm, thereby also contributingto the reduction in heat dissipation.

To further improve the heat transfer from the heater structure in thedirection of the sensor structure, it is advantageous when the secondinlay on the side of the heater structure facing away from the sensorstructure has a porosity or a porous void structure created inparticular with the help of a pore forming agent in the course of asintering operation used in the manufacture of the sensor element.Alternatively or additionally, the second inlay may also be providedwith cubical, cylindrical, or lenticular milled-out areas or recesses.

It is also frequently advantageous when the electrically insulatingfirst inlay used on the side of the heater structure facing the sensorstructure has a porosity or porous void structure created using a poreforming agent and/or when the first inlay has cubical, cylindrical orlenticular recesses, for example. Due to this structure of the firstinlay, the heat transfer from the heater structure in the direction ofthe sensor structure is initially somewhat hindered, but thisadvantageously further reduces the capacitive input of electric signalsfrom the heater structure into the sensor structure. Moreover, aporosity of the inlays or the provision of recesses therein is generallyadvantageous in order to reduce mechanical stresses.

To reduce mechanical stresses in the sensor element during manufactureand/or operation, in particular in the case of expansions, it is alsoadvantageous when the first and/or second inlay has at least one recess,which may be a plurality of cuts or slots traversing the inlay in someareas. These may be situated so that when seen from above, they are notabove or below an area occupied by the heater structure. To this end,these cuts or slots in the inlays are provided at those locations wherethe heater structure is not above or below them.

The use of comparatively thick inlays of aluminum oxide that are dense,i.e., not porous, as the first and/or second inlay has the advantagethat they result in particularly effective electric insulation of theheater structure with respect to the surrounding zirconium dioxidelayers and this also prevents platinum from diffusing from the heaterstructure into the layers. Moreover, inlays of aluminum oxide arecomparatively good thermal conductors, which improves the effectiveheating of the sensor structure.

It is also advantageous when the inserted first inlay and/or the secondinlay has a beveled edge as seen from above at least in the area of thetransition from the hot area of the heater structure into the cold areaof the heater lead wire, these bevels may be directed in oppositedirections in the case that the edge of the first inlay and the edge ofthe second inlay are both beveled. The bevels thus define, as seen fromabove, an overlap area in which there is a transition from the heaterstructure to the heater lead wires. Beveling the edges of the inlaysprevents the development and propagation of cracks in the inlays due tomechanical stresses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first exemplary embodiment of a sensor element accordingto the present invention having an embedded heater area above whichthere is a sensor structure.

FIG. 2 shows a longitudinal section of FIG. 1 in the heater area as seenfrom above.

FIG. 3 shows a second exemplary embodiment of a sensor element accordingto the present invention.

FIG. 4 shows another exemplary embodiment of a sensor element accordingto the present invention.

FIG. 5 shows an inlay of aluminum oxide having lenticular recesses.

FIG. 6 shows an inlay of aluminum oxide having a cubical recess.

DETAILED DESCRIPTION

FIG. 1 shows a sensor element 30 having a sensor structure 19 and aheater area 30′. With regard to the production of sensor element 30according to FIG. 1, known techniques are used, i.e., ceramic greenfilms onto which other layers are printed as needed, then stacked,laminated and finally sintered to form sensor element 30.

FIG. 1 shows in detail a sintered ceramic sensor element 30 in the formof a planar gas sensor element having a solid electrolyte including abottom layer or a substrate 5 of zirconium dioxide on which there is asecond intermediate layer 10 in some areas, printed onto the ceramicgreen film that forms substrate 5 at the time of its manufacture. Asecond spacer layer 4 of zirconium dioxide is also provided and forms alower frame, thus defining a trough-shaped second recess 16 into which asecond inlay 2 is placed after the printing of second intermediate layer10 and deposition of second spacer layer 4 onto substrate 5. In theexample described here, second inlay 2 is electrically insulatingfollowing the sintering operation that concludes the manufacture ofsensor element 30, and it has a thickness of 200 μm to 500 μm. Duringmanufacture, it is first inserted as a ceramic green film using analuminum oxide ceramic and is then converted into an aluminum oxideceramic by sintering in conjunction with the other components of sensorelement 30.

Then a heater structure 1 in the form of a platinum resistance conductoris applied, such as by printing, to some areas of second spacer layer 4and second inlay 2. The area above second inlay 2 defines a hot area 3of sensor element 30. In addition, conventional heater lead wires 6,which run on second spacer layer 4 and are also formed, for example, byprinted platinum conductors, are provided. Heater lead wires 6 run in acold area 8 of sensor element 30 and are separated from second spacerlayer 4 and second inlay 2, respectively, by an insulation layer 7,which may also be printed and is situated beneath heater lead wire 6. Toachieve a reliable connection of second spacer layer 4 and second inlay2 with insulation layer 7, a transition material 14 is provided in thearea of insulation layer 7, which is composed of a mixture of aluminumoxide and zirconium dioxide, for example, and may form a partial layerof insulation layer 7, so that insulation layer 7 and second inlay 2 andspacer layer 4, respectively, are reliably and fixedly joined together.

FIG. 1 also shows that an insulation layer 7 and a transition material14 are also provided in some areas on the side of heater structure 1facing away from heater lead wire 6 to achieve an electric insulation ofheater structure 1 from second spacer layer 4 and first spacer layer 4′,respectively, the first spacer layer being positioned above the secondspacer layer and being explained below. Heater structure 1 has awave-form design in the example explained here.

A first spacer layer 4′, which may have a similar design and is situatedon second spacer layer, is also made of zirconium dioxide, for example,and is also designed in the form of a closed frame. This first spacerlayer 4′ defines a first recess 15 into which a first inlay 9 ofaluminum oxide ceramic is inserted. Then a first intermediate layer 11is also applied over the entire area of first inlay 9 by printing it inthe course of manufacturing onto first inlay 9, which is initially inthe form of a ceramic green film. The composition of first intermediatelayer 11 may correspond to the composition of second intermediate layer10. The composition of first inlay 9 may be the same as the compositionof second inlay 2, i.e., after sintering it may also composed of analuminum oxide ceramic.

On the whole, first spacer layer 4′, second spacer layer 4 and firstinlay 9 enclosed by it laterally, second inlay 2 and first intermediatelayer 11 as well as second intermediate layer 10 define heater area 30′,the thickness of first intermediate layer 11 and second intermediatelayer 10 being selected so that together with inlays 2, 9 and heaterstructure 1, they completely and evenly seal recesses 15, 16 in spacerlayers 4, 4′.

According to FIG. 1, an insulation layer 7 having a transition material14 is also situated on heater lead wire 6 so that heater lead wire 6 isalso enclosed by insulation layer 7 and transition material 14 and isthus electrically insulated with respect to spacer layers 4, 4′ and insome areas also with respect to inlay 2, 9.

Another layer 17, which may be a zirconium dioxide layer, is providedover the entire area of first spacer layer 4′, and then the sensorstructure 19 is constructed on this layer so that sensor structure 19 isheatable by heater structure 1.

The design described here achieves the result that heater structure 1,which is enclosed on both sides by directly adjacent inlays 2, 9, iselectrically insulated with respect to sensor structure 19 via firstinlay 9, so that capacitive coupling is largely suppressed.

The thickness of first inlay 2 and/or second inlay 9 is between 200 μmand 500 μm. The thickness of first intermediate layer 11 and/or secondintermediate 10 is 5 μm to 50 μm, e.g., 10 μm to 30 μm.

The first and/or second intermediate layer 10, 11 is used mainly toabsorb or equalize mechanical stresses between first inlay 9 andadditional layer 17 and between second inlay 2 and substrate 5 thatoccur during sintering in the course of manufacturing sensor element 30.Therefore, first and/or second intermediate layer 10, 11 has a lowsintering activity during sintering with respect to the adjacent inlayand substrate 5 or additional layer 17, and does not sinter to a denseform, i.e., it remains porous, or first and/or second intermediate layer10, 11 becomes fused to adjacent inlay 9 and adjacent additional layer17 and adjacent second inlay 2 and adjacent substrate 5, respectively,in this sintering process.

With regard to the composition of first and/or second intermediate layer10, 11, it is advantageous when it contains at least one elementselected from the group of aluminum, magnesium, zirconium or barium.Both first and second intermediate layer 10, 11 may be composed eitherof a magnesium aluminum spinel, such as MgAl₂O₄, barium hexaaluminate,or a mixture of zirconium dioxide and aluminum oxide.

The lateral extension of second recess 16 filled by second inlay 2 andfirst recess 15 filled by first inlay 9 is may be large enough to coverthe area taken up by hot area 3 of heater structure 1 as seen fromabove.

FIG. 2 shows a longitudinal section of FIG. 1 in heater area 30′. Thisshows only heater structure 1, heater lead wire 6 including insulationlayer 7, which is located beneath it, and transition material 14, aswell as second inlay 2 which is above or below heater structure 1 in hotarea 3 and first inlay 9. This shows clearly the meandering structure ofheater structure 1 in hot area 3 and comparatively wide heater lead wire6 in comparison with the width of the actual heater structure 1, whichis designed in the form of a platinum resistance conductor.

In a continuation of FIG. 1, FIG. 2 also shows that first inlay 9 andsecond inlay 2 also each have a beveled edge 12, 13, as seen from above,in an overlap area 26, which also defines a transition from hot area 3to cold area 8, the bevels of these two edges 12, 13 being directed inopposite directions to one another. The shape of first recess 15 infirst spacer layer 4′ is therefore designed according to the shape offirst inlay 9 and the shape of second recess 16 in second spacer layer 4is designed according to the shape of second inlay 2 according to FIG.2.

FIG. 2 shows that second inlay 2 and/or first inlay 9 may optionallyhave recesses 7′ in the form of slots or cuts. These recesses 7′ aresituated in such a way that they are not above or below an area coveredby heater structure 1 as seen from above. FIG. 2 also clearly shows arear area 25 formed by first spacer layer 4′ and second spacer layer 4beneath the first spacer layer. This rear area 25 is much wider than 300μm, e.g., 500 μm to 2000 μm.

FIG. 3 illustrates an exemplary embodiment of a sensor element 30 as analternative to that in FIG. 1 or the variant according to FIG. 2, secondinlay 2 being designed as a second inlay having a porous void structure2′ to better absorb, i.e., dissipate, mechanical stresses in this way.The porous void structure is achieved by first adding an additional poreforming agent to the ceramic green film, which is designed as an inlayor intarsia, and which forms the second inlay having a hollow structure2′ after sintering, so that in the course of the sintering process,second inlay 2′ develops a porous void structure. Pore forming agentssuitable for this purpose, such as carbon black particles or glasscarbon particles, are known from the related art.

In a continuation of FIG. 3, FIG. 4 illustrates another exemplaryembodiment of a sensor element 30, first inlay 9 also being designed inthe form of a first inlay having a porous void structure 9′. The secondinlay having a porous void structure 2′ is designed in FIG. 4 accordingto FIG. 3. This achieves the result that the capacitive coupling ofheater structure 1 in the area of sensor structure 19 is further reducedand mechanical stresses are further reduced or better absorbed ordissipated. However, the produced hollow structure may result in theheat transfer from heater structure 1 into the area of sensor structure19 being less effective.

FIGS. 5 and 6 illustrate other exemplary embodiments of first inlay 9,second inlay 2 also being able to be designed in the same way. Inparticular, FIG. 5 shows how first inlay 9 is provided with lenticularrecesses 20, preferably on the side of inlay 9 facing heater structure1, instead of the porous void structure according to FIG. 4. Otherwiseinlay 9 is again composed of aluminum oxide ceramic. Lenticular recesses20 are created for example via corresponding milling of the ceramicgreen film used initially to manufacture first inlay 9. According toFIG. 6 a cubical recess, i.e., milled-out area 21, is provided.

1-16. (canceled)
 17. A planar gas sensor element comprising: a heaterstructure; a sensor structure heatable by the heater structure; and afirst spacer layer situated between the heater structure and the sensorstructure; wherein the first spacer layer includes a first recessproximal to the heater structure and a first inlay inserted into thefirst recess, the first inlay electrically insulating the heaterstructure from the sensor structure.
 18. The sensor element of claim 17,further comprising: a substrate situated on a side of the heaterstructure facing away from the sensor structure; and a second spacerlayer situated between the heater structure and the substrate; whereinthe second spacer layer includes a second recess proximal to the heaterstructure and a second inlay inserted into the second recess, the secondinlay electrically insulating the heater structure from the substrate.19. The sensor element of claim 18, wherein the first and the secondrecess have approximately the same dimensions and are positionedapproximately in a stacked arrangement, and wherein the heater structureis positioned between the first inlay and the second inlay.
 20. Thesensor element of claim 18, wherein at least one of the first and secondinlay is a ceramic film.
 21. The sensor element of claim 20, wherein theceramic film includes one of an aluminum oxide film and a filmconvertible into an aluminum oxide film by sintering, the aluminum oxidefilm having a thickness of 100 μm to 1000 μm.
 22. The sensor element ofclaim 21, wherein the aluminum oxide film has a thickness of 200 μm to500 μm.
 23. The sensor element of claim 18, wherein at least one of: a)the first spacer layer surrounds the first recess laterally in the formof a closed frame; and b) the second spacer layer surrounds the secondrecess laterally in the form of a closed frame.
 24. The sensor elementof claim 18, wherein the first spacer layer and the second spacer layerare situated one above the other, and the heater structure separates thefirst inlay from the second inlay in at least one area.
 25. The sensorelement of claim 18, further comprising: at least one heater lead wire;at least one insulating layer, the insulating layer insulating the atleast one heater lead wire from the first and second spacer layers; andan additional layer; wherein the substrate, the first spacer layer, thesecond spacer layer and the additional layer enclose the first andsecond inlays and the heater structure with the exception of the atleast one heater lead wire and the at least one insulating layer. 26.The sensor element of claim 25, further comprising: a) firstintermediate layer configured to provide electric insulation, situatedbetween the first inlay and the additional layer in at least one area;and b) a second intermediate layer configured to provide electricinsulation, situated between the second inlay and the substrate in atleast one area.
 27. The sensor element of claim 26, wherein the firstintermediate layer is configured to at least one of absorb and equalizemechanical stresses between the first inlay and the additional layer,and wherein the second intermediate layer is configured to at least oneof absorb and equalize mechanical stresses between the second inlay andthe substrate.
 28. The sensor element of claim 27, wherein themechanical stresses are associated with at least one of sintering and achange in temperature during operation.
 29. The sensor element of claim26, wherein at least one of: the first recess is filled completely andevenly with one of i) the first inlay and ii) the first inlay and thefirst intermediate layer; and the second recess is filled completely andevenly with one of i) the second inlay and ii) the second inlay and thesecond intermediate layer.
 30. The sensor element of claim 25, whereinat least one of the first inlay and the second inlay includes a bevelededge in an area where a hot region of the heater structure transitionsto a cold area of the heater lead wire.
 31. The sensor element of claim30, wherein in the case where both the first inlay and the second inlayinclude a beveled edge, the bevels are directed in opposite directions.32. The sensor element of claim 26, wherein at least one of the firstand the second intermediate layers includes one of the following: i) atleast one element selected from the group of Al, Mg, Zr and Ba; ii) anMg—Al spinel; iii) barium hexaaluminate; and iv) a mixture of zirconiumoxide and aluminum oxide.
 33. The sensor element of claim 32, whereinthe Mg—Al spinal is MgAl₂O₄.
 34. The sensor element of claim 25, whereinthe first spacer layer, the substrate, the second spacer layer and theadditional layer include zirconium oxide.
 35. The sensor element ofclaim 18, wherein the first spacer layer surrounds the first recess inthe form of a closed frame, and the second spacer layer surrounds thesecond recess in the form of a closed frame, each closed frame having awidth greater than 300 μm, and wherein a rear area of a heater area isformed by the first and second spacer layers.
 36. The sensor element ofclaim 25, wherein the width of each closed frame is between 500 μm and2000 μm.
 37. The sensor element of claim 18, wherein at least one of thethe first inlay and the second inlay has a porous structure formed usinga pore forming agent in the course of a sintering process and one of acubical, cylindrical and lenticular milled-out area.
 38. The sensorelement of claim 18, wherein at least one of the first inlay and thesecond inlay has at least one of a recess, a cut and a slot that is notvertically aligned with an area covered by the heater structure.